Suspensions of coal and coal by-products in fuel oil have been proposed as a fuel for blast furnaces, boilers and other fuel burning devices, but because of the danger and difficulty in handling and maintaining the suspensions over long periods of time, no practical system has yet been devised for the large scale use of this combination fuel. Methods have been proposed that require the use of an expensive emulsifier, which severely reduce the economic benefits of the system.
U.S. Pat. No. 2,118,477, issued May 24, 1938 to Roberts teaches the combination of colloidal coal particles and oil to form a suspension for use as a composite fuel. The difficulty, expense, and hazards in grinding coal to colloidal fineness, such that it passes through a 300 to 1,000 mesh sieve, has prevented the composition fuel disclosed by Roberts from being used in present day furnaces.
U.S. Pat. No. 2,231,513, issued Feb. 11, 1943 to Stillman, teaches a stable suspension of coal and oil made by pre-grinding coal particles until they pass through a 100-mesh screen and thereafter introducing the pre-ground particles into oil and subjecting the combination to a further grinding until the particle size is reduced to about 5 microns. All of the coal particles are small enough to pass through a 325-mesh screen, and at least 50% of the particles are under 10 microns in size, which is sufficient for the particles to produce the phenomenon known as the "Brownian Movement". The "Brownian Movement" of the particles is sufficient to maintain all of the coal particles in stable suspension, in lieu of starch, since starch forms a "gel" with a disadvantageously high viscosity making preheating and pumping the slurry difficult.
Other parent, such as the Plauson et al. U.S. Pat. No. 1,647,471, issued Nov. 1, 1927, teach the combination of pre-ground carbonaceous material with an oil. The Plauson et al. patent teaches pre-grinding the carbonaceous material to a powder which passes through a 125 to 250-mesh screen, and, thereafter, forming an emulsion with the oil by the addition of from about 1% to about 3% of a soap solution. The Plauson et al. patent has an additional disadvantage besides the small particle size of the carbonaceous material as the suspension or emulsion is produced in small batches using a cross hammer mill rotating at 325 feet per minute.
U.S. Pat. No. 2,090,393, issued Aug. 17, 1937 to A. A. Roberts, teaches the use of a minor amount of starch in the order of 150 of 1%, mixed with water and an electrolyte such as a boron compound. The fundamental thrust of the patent is to improve combustion, not to produce a slurry stable for several weeks. The specific problems to which the present invention is directed are not discussed in the Roberts patent.